Recent studies in this laboratory demonstrated that several sulphated polysaccharides can inhibit metastasis of the rat mammary adenocarcinoma 13762 MAT, probably by preventing the passage of tumour cells through the walls of blood vessels. In order to directly test this possibility, 13762 MAT cells were cultured with (³⁵S)O₄⁼‐labelled subendothelial extracellular matrices (ECM) and ECM degradation was monitored in either the presence or absence of different sulphated polysaccharides. Degradation products were detected by sodium dodecyl sulphate polyacrylamide gel electrophoresis and subsequent autoradiography. The 5 sulphated polysaccharides that had previously been shown to possess anti‐metastatic activity were potent inhibitors of the degradation of subendothelial ECM by 13762 MAT cells. In contrast, of the 4 polysaccharides tested that failed to inhibit metastasis, 3 had no effect on ECM breakdown and one (carrageenan‐kappa) was substantially less effective at inhibiting ECM degradation than the anti‐metastatic preparations. It was also shown that 13762 MAT cells produce a heparan sulphate‐specific glycosidase (heparanase) that degrades the heparan sulphate side‐chains of the ECM, the action of this enzyme rather than that of other ECM‐solubilizing enzymes being inhibited by the anti‐metastatic sulphated polysaccharides. Additional experiments indicated that the anti‐coagulant activity of the polysaccharides probably plays a minor role In their anti‐metastatic effects since heparin, almost completely depleted (98–99.5%) of heparin molecules with anti‐coagulant activity by passage over an anti‐thrombin III column, retained its ability to inhibit 13762 MAT heparanases and was almost as effective as unfractionated heparin at inhibiting tumour‐cell metastasis. Collectively, these data suggest that sulphated polysaccharides inhibit the metastasis of 13762 MAT cells by inhibiting tumour‐cell‐derived heparanases involved in the penetration of the vascular endothelium and Its underlying basement membrane by tumour cells.